The present invention relates to a transponder system for a motor vehicle.
Motor vehicles are often furnished with an anti-theft security system, which uses a transponder system to determine when a vehicle immobilizer should be activated. A transponder, having stored in it an access identifier, is built into the ignition key of the motor vehicle. The ignition lock contains an antenna, which can be used for transmitting signals to the transponder and for receiving signals from the transponder if the key is inserted into the lock. The antenna is connected to the on-board electronics module (BEM) of the motor vehicle. The BEM transmits a query signal to the transponder, prompting the transponder to transmit the access identifier to the BEM. If the received access identifier is valid, then the BEM deactivates the vehicle immobilizer. The BEM queries the transponder at regular intervals as to the valid access identifier, and if it is not received, if, for example, the ignition key is no longer in the motor vehicle, then the BEM activates the vehicle immobilizer.
Although the system described above significantly increases the anti-theft protection of the motor vehicle, a problem has nevertheless arisen. Third parties having access to the ignition key, such as an auto dealer or an employee of a hotel valet parking service, may be able to remove the transponder from the ignition key without the knowledge of the vehicle owner. They then attach the transponder to the steering column or to the ignition lock housing, so that it is within the range of the ignition lock antenna but so that it cannot be seen by the driver. The ignition key or a copy of it is then returned to the owner, who naturally does not suspect that the transponder has been removed. The vehicle immobilizer is thereafter never activated because the transponder is always present. Therefore, it is possible for third parties to thereafter steal the motor vehicle using conventional techniques such as breaking in short-circuiting and the ignition system. A transponder system which overcomes this problem, or at least represents a viable alternative, is therefore desirable.
The present invention relates to a transponder system for a motor vehicle, in which the transponder transmits an access signal as a response to a query signal. A signal processing device is provided for transmitting the query signal and for receiving the access signal, which the transponder transmits in response to the query signal. The signal processing device transmits the query signal as a transponder check signal, if the motor vehicle is deactivated, a warning signal being generated if the access signal is received as a response to the transponder check signal.